This invention is related to the production of metal by electrolysis in a molten bath. More particularly, the invention involves a method of preparing a cell for efficient operation in the electrolytic production of metal so that current losses and corrosion damage in the cell are reduced during such operation.
One method of operating a metal producing electrolysis cell to which the invention may be applied is described in U.S. Pat. No. 3,822,195 of Dell et al. This method operates to produce metal from the electrolytic reduction of the metal chloride dissolved in a molten solvent by electrolyzing the chloride-solvent bath in a cell which includes an anode, at least one intermediate bipolar electrode and a cathode. These cell elements are arranged in a superimposed, spaced relationship defining inter-electrode spaces between the anode and the uppermost electrode, between each pair of intermediate electrodes, and between the lowermost electrode and the cathode. In the practice of the method of Dell et al., electrolysis of the chloride-solvent bath takes place in each inter-electrode space to produce chlorine on each anode surface thereof and metal on each cathode surface thereof. The method also includes the establishment and maintenance of a flow of bath through each inter-electrode space to remove the metal produced from each such space. The bath flow is directed into, across and out of each inter-electrode space by utilization of the chlorine produced as the lifting gas in a gas lift pump which lifts the lighter bath upwardly while permitting heavier molten metal swept from each inter-electrode space to settle in a sump in the bottom of the cell.
The cell of Dell et al. includes an outer shell of steel which is lined with refractory brick made of thermally insulating, electrically nonconductive material. In the bottom of the cell is a graphite lined refractory sump for collecting the metal produced, and in the upper zone of the cell is a bath reservoir.
As series of improvements to the cell of Dell et al. is described in U.S. Pat. No. 4,110,178 of LaCamera et al. These improvements include the provision of an outer cooling jacket surrounding the shell of the cell, and the addition to the shell of an inner corrosion resistant, electrically insulating barrier portion. The shell of LaCamera et al. thus includes a steel portion having a coating of plastic or rubber material on the inside surface thereof and a layer of glass between this coating and the inside lining of refractory brick. An additional lining of graphite is positioned on the brick side walls alongside and above the anodes of the cell to provide further protection against the corrosive influence of the bath and the chlorine gas produced by the operation of the cell.
The preferred bath that is utilized in the production of metal in these cells is an essentially homogeneous solution comprised of the metal chloride dissolved in a molten solvent of higher decomposition potential. The method of Dell et al. and the cells of Dell et al. and LaCamera et al. are particularly appropriate for use in the production of aluminum. For such production, therefore, the bath composition normally is composed essentially of aluminum chloride dissolved in one or more halides of higher decomposition potential than aluminum chloride. These halides will usually be made up of alkali metal chlorides, although other alkali metal halides and alkaline earth halides may also be employed. A presently preferred composition of this production bath includes an alkali metal chloride base composition made up of about 50-75% by weight sodium chloride and 20-50% by weight lithium chloride. Aluminum chloride is dissolved in such base composition to provide a bath from which aluminum may be produced by electrolysis, and an aluminum chloride content of about 1.5-10% by weight of the bath will generally be desirable.
The relative concentrations of the two major bath constituents, sodium chloride and lithium chloride, exert the greatest influence on the electrical resistance of the production bath. Thus, for example, a higher concentration of sodium chloride results in higher resistance, and a higher concentration of lithium chloride results in lower resistance. As the electrical resistance of the production bath increases, the electrical efficiency of the cell operation decreases. Thus, with higher bath resistance, higher electrical current levels are required to produce a given amount of metal. For this reason, presently preferred production bath compositions include concentrations of sodium chloride at the lower end of the range described above, or in other words, near 50% by weight. As an example, a satisfactory production bath may be composed of 51% by weight sodium chloride, 40% lithium chloride, 6.5% aluminum chloride and 2.5% magnesium chloride. Traces of other chlorides, such as potassium chloride and calcium chloride, may be included as well, although the chlorides other than sodium chloride, lithium chloride and aluminum chloride may be regarded as incidental components or impurities. The production bath is maintained in a molten state, usually at a temperature above the melting point of aluminum, typically at about 700.degree. C.
In addition to the effect on electrical efficiency of the relative concentrations of sodium chloride and lithium chloride in the production bath, it is also known that these relative concentrations affect the solidus temperature, the temperature below which only the solid phase can exist, of substantially all of the bath. A higher concentration of sodium chloride will result in a higher solidus temperature for the bath. Conversely, a higher concentration of lithium chloride in the bath will result in a lower solidus temperature.
It has been found that, during the operation of the cells of Dell et al. or LaCamera et al., the production bath may impregnate the porous graphite linings, as well as the porous refractory linings in the cell. In addition, the bath may seep through the interstices between the refractory bricks, or through cracks in individual bricks that may arise from thermal stresses. Because the molten production bath is a good conductor of electricity, impregnation of the cell linings by the bath may provide an electrical pathway from the anode through the saturated linings to the cathode. The establishment of such a pathway would result in a current flow that would bypass the intermediate bipolar electrodes, and thus would not contribute to the production of metal. Such a current loss reduces the efficiency of cell operation because when it exists, a greater current level must be introduced to the cell to produce a given amount of metal.
In addition, although an electrically insulating, corrosion resistant barrier portion may be provided between the steel portion of the shell of the cell and the refractory linings, occasional failure of this barrier portion may lead to contact between the bath and the steel portion of the shell, thus establishing an electrical pathway of molten bath between the electrically conductive steel portion of the shell and the anode or cathode of the cell. Since the cells of Dell et al. and LaCamera et al. are of the type employing bipolar electrodes, zones of opposite polarity may be established in the steel portion of the shell by contact therewith by the liquid bath. If this happens, localized electrolysis of the bath may occur at these polarized zones in the steel portion of the shell. Such localized electrolysis may impair cell operation by depositing metal at negatively polarized, or cathodic zones in the steel portion. The metal thus deposited on the steel portion may alloy with the steel and thereby weaken the shell. Further characteristic of this localized electrolysis in a bath comprised of a metal chloride dissolved in a molten solvent of higher decomposition potential is the evolution of chlorine at positively polarized, or anodic zones in the steel portion of the shell. Chlorine is highly corrosive to the steel in the shell, and may cause deterioration and even perforation of the shell at sites of chlorine evolution. Perforation of the shell could be particularly disastrous if it led to contact of the bath with a coolant such as water which may be provided in a cooling jacket on the exterior of the shell.
Another problem may arise during the operation of the cells of Dell et al. or LaCamera et al. when these cells are constructed with refractory brick linings containing silica. This problem involves a reaction between the aluminum metal produced in the cells and the silica in the brick. As electrolysis proceeds in these cells, droplets of produced metal may impregnate the porous brick lining. When this occurs, the aluminum metal may react with the silica in the brick to form alumina and silicon. Such a reaction causes rapid deterioration of the structure of the brick, by the removal of silica therefrom.